1. Field of the Invention
The present invention relates to a transparent display device, and more particularly, to a transparent display device in which luminance in an image mode is enhanced.
2. Description of the Related Art
In recent years, with rising interests in information displays and increasing demands to use portable information media, researches and commercialization of light-weight and thin-profile flat panel displays (FPDs) for substituting traditional displays such as cathode ray tubes (CRTs) have been actively carried out. In particular, among such FPDs, a liquid crystal display (LCD), which is a device displaying images using an optical anisotropy of liquid crystal molecules, has been actively applied to a notebook, a desktop monitor, or the like, because it is excellent in the resolution, color representation, image quality, and the like.
On the other hand, studies on a transparent display device for allowing rear objects thereof to be seen as well as capable of implementing images thereon have been actively carried out. Such a transparent display device may be applicable to vehicle front glasses or house glasses to provide the user's desired information. Therefore, the applicability of such transparent display devices may be expected to be drastically increased.
In general, it may be used an organic light-emitting display device and the like using spontaneous light for the transparent display device.
However, in case of the organic light-emitting display device, a display device thereof can be made only to be transparent, and thus it may be impossible to turn on or off the transparency to make it transparent or implement an image thereon. Also, there are various problems such as low yield, difficulty in making a large-sized display, low reliability, and the like.
As a result, it may be required to develop a liquid crystal display device capable of implementing high yield, large-sized displays, high reliability, as well as capable of implementing wide viewing angle, high luminance, high contrast ratio and full color as a transparent display device, but the liquid crystal display device cannot be used as a transparent display device. However, the liquid crystal display cannot spontaneously emit light but implement an image by using light of the backlight because a non-transparent backlight unit should be provided at a rear surface of the liquid crystal display panel and also polarizing plates should be provided at both front and rear surfaces of the liquid crystal display panel, respectively, to control the transmission of light. In particular, the polarizing plates provided at both front and rear surfaces of the liquid crystal display panel, respectively, allows light to be transmitted therethrough when liquid crystals are driven in the liquid crystal display panel, but light is in a non-transparent state when liquid crystals are not driven, and thus it is impossible to implement a transparent display.